A known signal processing unit of the above type may be a status unit, a control unit or a combined status-and-control unit.
A status unit monitors the state of one or more input signals and when a change of state occurs at a status input a message to that effect is generated and sent to the central station, the message containing an address which identifies the transmitting unit.
A control unit provides one or more control outputs at which signals are provided for remote control functions. Activation of a control output may result in the closure of a relay contract and this activation is initiated by means of a message sent from the central station to the control unit.
A status-and-control unit combines the functions of the other two types of unit.
A problem with known signal processing units of the above type is their inability to adapt to changing environmental conditions. For example, transmission may become unreliable due to transmission path deterioration.
This invention seeks to provide a signal processing unit which is more adaptable than known units and in which the above-mentioned problem is at least mitigated.